1. Field of the Invention
The present invention relates generally to cementing subterranean wells, and more particularly, to cement compositions which set into resilient impermeable solid masses and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior surfaces of the pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The cement compositions utilized in primary cementing must often be lightweight to prevent excessive hydrostatic pressures from being exerted on formations penetrated by well bores. A particularly suitable technique for making a hydraulic cement composition lightweight is to foam the cement composition with a gas such as air or nitrogen. In primary cementing, a foamed cement composition provides the additional advantage of being compressible whereby formation fluids are less likely to enter the annulus and flow through the cement composition therein during the transition time of the cement composition, i.e., the time after the placement of a cement composition in the annulus during which the cement composition changes from a true fluid to a hard set mass.
The development of wells including one or more laterals to increase production has recently taken place. Such multi-lateral wells include vertical or deviated (including horizontal) principal well bores having one or more ancillary laterally extending well bores connected thereto. Drilling and completion equipment has been developed which allows multiple laterals to be drilled from a principal cased and cemented well bore. Each of the lateral well bores can include a liner cemented therein which is tied into the principal well bore. The lateral well bores can be vertical or deviated and can be drilled into predetermined producing formations or zones at any time in the productive life cycle of the well.
In both conventional single bore wells and multi-lateral wells having several bores, the cement composition utilized for cementing casing or liners in the well bores must develop high bond strength after setting and also have sufficient resiliency, i.e., elasticity and ductility, to resist loss of pipe or formation bond, cracking and/or shattering as a result of pipe movements, impacts and/or shocks subsequently generated by drilling and other well operations. The bond loss, cracking and/or shattering of the set cement allows leakage of formation fluids through at least portions of the well bore or bores which can be highly detrimental.
The set cement in a well, and particularly the set cement forming a cement sheath in the annulus between a pipe string and the walls of a well bore, often fails due to shear and compressional stresses exerted on the set cement. Such stress conditions are commonly the result of relatively high fluid pressures and/or temperatures inside the cemented pipe string during testing, perforating, fluid injection and/or fluid production. The high internal pipe pressure and/or temperature results in the expansion of the pipe string, both radially and longitudinally, which places stresses on the cement sheath causing it to crack or the cement bonds between the exterior surfaces of the pipe and/or the well bore walls to fail whereby the loss of hydraulic seal in the annulus occurs.
Another condition results from exceedingly high pressures which occur inside the cement sheath due to the thermal expansion of fluids trapped within the cement sheath. This condition often occurs as a result of high temperature differentials created during the injection or production of high temperature fluids through the well bore, e.g., wells subjected to steam recovery or the production of hot formation fluids from high temperature formations. Typically, the pressure of the trapped fluids exceeds the collapse pressure of the cement and pipe causing leaks and bond failure.
Yet another compressional stress condition occurs as a result of outside forces exerted on the cement sheath due to formation shifting, overburden pressures, subsidence and/or tectonic creep.
In multi-lateral wells wherein pipe strings have been cemented in well bores using conventional well cement slurries which set into brittle solid masses, the brittle set cement cannot withstand impacts and shocks subsequently generated by drilling and other well operations carried out in the multiple laterals without cracking or shattering.
The above described failures can result in loss of production, environmental pollution, hazardous rig operations and/or hazardous production operations. The most common hazard is the presence of gas pressure at the well head.
Thus, there are needs for improved well cement compositions and methods whereby after setting, the cement compositions are highly resilient and can withstand the above described stresses without failure. That is, there is a need for well cement compositions and methods whereby the cement compositions have improved mechanical properties including elasticity and ductility and failures due to pipe movement, impacts and shocks are reduced or prevented.